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Skeletal muscle consists of two types of myocytes:
The ratio of skeletal muscle cells is mainly determined by genetics, and athletic potential of each person is largely depends on it.
Each muscle cell consists of many myofibrils - thin filaments of protein (actin and myosin) that can make contractions. Due to massive contraction of myofibrils the contraction of whole muscle is performed.
|Type of fibers||Speed of contraction||Capacity for growth (hypertrophy)||Speed of fatigue||Used for||Strenth||Number of mytochondria||Blood supply||Oxydative capacity||Glycolytic capacity||Source of energy|
|Type I (slow)||Slow||Low||Low||aerobic activity (running, cycling)||Low||Many||Abundant||High||Low||Fats|
|Type IIа (transitional)||Fast||Low||Moderate||Prolonged anaerobic load||High||Many||Moderate||High||High||Creatine phosphate, glycogen|
|Type IIb (fast)||Very fast||High||High||Short anaerobic (strength training)||Very high||Few||Poor||Low||High||Creatine phosphate, glycogen|
Quick muscle fibers (aka white) use anaerobic (oxygen-free) metabolism in the production of energy for contraction. They perform high-speed movements, which are characterized by large explosive power, but they get tired much earlier than slow ones. Both cell types produce roughly the same amount of work in a single movement, but white cells do that much faster.
There are two types of white muscle fibers: IIa and IIb.
Cells of IIa subtype are also known as intermediate or transitional. They can use both aerobic (accompanied by oxygen consumption) and anaerobic (anoxic) metabolism for energy production equally. These fibers are in the middle between fast and slow ones.
They are true fast muscle fibers, which can use only anaerobic metabolism, having maximum of strength and speed of contraction. These cells play major role in gaining mass in bodybuilding, so almost all the training programs are designed for development of this type of fibers.
White IIb fibers can get hypertrophic to a greater extent than slow ones.
It is this type of cells which makes a major contribution to achievement in those sports that require explosive power:
Scientists have found that the ratio of fast and slow muscle fibers is genetically determined. The average person has almost equal ratio. The best results in bodybuilding are achieved by those athletes whose muscles contain more white fibers.
White muscle fibers are also important for sprinters, as fast muscle fibers always prevail in outstanding sprinters (about 80%).
There is evidence that featured workouts can affect this ratio. Strength training in bodybuilding can increase the number of cell of IIa & IIb types, while aerobic exercises increase the content of slow twitch cells (type I). However, these changes are quite limited. In studies it was discovered that transition from one type to another usually does not exceed 10%. For this reason, some people gain muscle mass with great difficulty, while others, on the contrary, very quickly.
If you want to get a fine figure and look attractive, this can be achieved even with white fibers in minority. Unfortunately, your achievements in professional sports will be small.